u/My_rune_rock

I want to fill this area but there are no crisp flat edges and being ply i cant do a lot to flatten it out, what are my best options ?
Ideally it would work to get everything taped, clamped and stuck down and use a cotton bud to wipe around the edge with vasaline to fill the gaps but no idea if that will actually work??
Second to that i could do the same with wood filler and let it set in place but messier and longer..

Edit: Its worth a mention that the facedown side will be the display side once finished so it needs to be somewhat easy to clean up or cover over