Awesome pond ....that was the same exact idea I had to the T....8x4 plywood and 2x4s all around....
I also thought about getting a larger piece of pond liner cutting extra off and have it drape over both sides to hide the folds and prevent the rays from biting thru
imo liners and rays are risky buisness... liners and any fish with teeth is risky buisness. o.p. i like your idea of using additional liner pieces draped on the sides to cover the folds. id also recommend tiles for the bottom. 45mil epdm is not that strong...plenty videos of a guy poking a pen through very easily lol... as others have said there are 60mil liners and also new RPE (reinforced poly ethelyene) liners that are more puncture resistant than epdm and still around the same price. i build liner tanks/ponds with windows and recently had some fish chew through on a 1400gal with 45mil epdm. i plan 2 use 1/8" plastic sheeting ontop of my liner on every surface after putting a new one in. 45 mil Epdm's are pretty tough, but theyll last alot longer with a bit of extra assurance in "toothy critter prevention" lol...
At this point to help me sleep at night it might be better if I used an intex pool with 60 mil pond liner on top and just frame the pool off for looks....at least I'll have 2 layers of water proofing