New Kodiak Livewell in the 20SF!

[gofastsandman]

Help is appreciated,

Picked up a used oval 40 gal livewell and am planning on a stepped drain to allow me to run 20 , 30 or 40 gals depending on num of people and amount/type of bait.I have an 800gph pump for free flow and picked up another for recirc/ washdown duty.
Now to the questions.
1/ should the ricirc diffuser be as low as poss and w/ tiny holes for O2 infusion?

2/How close to the top of the well can I put the drain? It has a round 12" recessed hole w/ a downward lip in the middle

3/is there a way to prevent siphoning w/ out a check valve?

4/ who makes a good diffuser?

5/ it`s a poly tank. how do I seal the fittings?

Thx,
GFS
ps Nice Kodiak. Wish I didn`t have so much Scottish/ cheap blood cursing through my veins.