Harry: Stick with the PVC structure. Many times old Christmas trees and the like were used for this, but recent research is finding that there are detrimental side effects to this. I don't remember what they are, just that trees are not a very good choice. Some rubble or broken up concrete here and there might not be a bad idea, especially in the shallow areas. A concrete contractor would be happy to furnish some of this for you! If you live where ice gets thick enough to allow you to walk on it, the structure can be placed on the ice just before ice out. Tom

 Personally have no formal education in biology or aquatic training, most of my previous experience has been done in the early AM, generally would prefere the cover of bad weather to pursue my rather strange "hobby" and no; the majority of those times did not have "official" permission to do so. Having said that, DO now have OFFICIAL permission to place 1,000+ fish attractors in CJ Brown Reservoir inside Buck Creek State Park in Ohio. That permission is from the US Army Corps of Engineers, Ohio Dept of Wildlife and Ohio Dept of Parks; to date we (the Friends of CJ Brown Reservoir) have placed 651+ in 27 groups all over that body of water, GPSed them and made their positions known and available to the general fishing public. We have agreed to follow some requirements for permission to do this, which is acceptable to us. A few suggestions; NORMAL concrete blocks are generally biologically INERT and should be OK. In shallower water stuff branches THRU the blocks in "opposed " (all 1 way 1 side, all opposite direction other side) method, secure with light steel cable or VHD weed eater line. "C" (Christmas) trees work well, but flatten over time; in gravel pit/ quarries (infertile to moderately fertile bodies of water) they should last 5-8? yrs; in more fertile waters 3-5?yrs. Would recommend in order CEDAR, CYPRUS, HEDGE APPLE (OSAGE ORANGE), LOCUST, MULBERRY, HICKORY, and OAK* (a few states actually forbid sinking oak WITH leaves on it)  PVC is useful but because it isn`t organic and obviously doesn`t decay  it doesn`t attract and harbor NEARLY the wide range of bacteria, algea, insect larvea, crayfish, minnows, shad, ect that you get with organic materials, and since there are only rounded surfaces there is little to NO cover for said diverse lifeforms to hide or spawn on/ in. The more dense the cover is the more successful they will be, generally. Also, when sinking them, placing them in rows shallow to deep (6-20 fow) SEEMS to have the greatest effect; fish use them as a stairway; TRANSITION ZONES where the type of bottom changes from riprap to say muck? sand, or gravel are EXCELLENT spots to "splash " them into. Place them in sets of atleast 5-10; on level bottoms a rough circle works well.  Also try piles of 10 hole bricks, you just won`t BELIEVE the # of crayfish they will attract- have actually SEEN bass come up to them, eyeball the holes and finding one sheltering a small mudbug, carefully align themselves and suck them out, just like a straw! A couple of other points; try to secure some trees horizontally, perch will spawn on them, bass and cats and crappie will all build nests under the cover. And USE the stronger attachment materials, alot of time cutting off the lower limbs on one side allows for a MUCH stronger connection; ensure it`s tight by lifting the tree/ or limbs; if there`s slack RETIE it. Both should lift together with little if any slack. And as far as how long they take to work...afew days at most IF NOT HOURS! DO keep in mind though, as someone else mentioned, YES they are more effective once the needles/ leaves fall off- generally they are best their 2cnd yr underwater. That said will give you an example then will close; in the Marina CJ Brown  the Park has a kid`s fishing program every Sat 10AM to Noon, from Memorial Day to Labor Day. Last yr they caught 21 fish in  the first 8 wkends. The Park Manager, who previously was adamant NONE would go into the Marina itself said he wanted the fishing results in the Marina to improve. Suggested stocking  the lake. (He DID NOT think that was FUNNY AT ALL!) He "suggested dropping afew" where the kids usually fished. In went 36 cedar brush blocks (BBs) Next Sat the kids caught 43 fish in 2 hrs.  Coincedense?( We are preparing to place 100+ into Marina before ice up.) Give something back to whatever body of water you fish, although would STRONGLY recommend getting OFFICIAL permission IF you are planning on placing them in PUBLIC waters. "...IF YOU ONLY KNEW THE POWER OF THE DARK (deeper) SIDE OF THE FISH ATTRACTORS..." Respectfully, the Friends of CJ Brown Reservoir.