I have a 10’ gun (10.5x21x10.5) in a tri-fin set up. It is a nice board but it tracks, way too stiff. (I have a 10’6 that flows much better in a single fin) I was thinking of changing the tri-fin to a 2+1 with side bites. Is this a bad idea? Three issues come to mind-1) shape of bottom might not accommodate the change in fin set-up 2)what are the challenges of ripping out Future boxes, dropping in a standard single fin box and creating a good watertight seal (post glassing installation) 3) object being to create a more fluid board, should I just try other future fins (current are "yu"s).
I built a board in 2008. Called it a 7'2" California gun. I got fin placements from Swaylock's.com...........Board was way too stiff...my bad...I asked for gun placement when I really wanted an all around board.......One year later I cut out all the boxes and made a 2 plus1 set up. No problem....you can see where the old boxes use to be...I can live with that... That's the great thing about making your own boards....If it needs to be modified...do it!
Cut out the old boxes.....install foam from old broken surfboards...sand smooth....Lay out new fin set up...install new boxes. Glass over the whole thing.....go surfing!
You may not need to fill holes with foam leftovers: routing out the center box for a FU fin-box installation is no problem since the FU box is bigger than the Futures box and you may keep side boxes and just change the current fins for side bites (SB1 or SB3 on Futures catalog). Start with that and see if the toe-in angle has to be modified or not. That will save you a lot of work.
Standard single fin boxes - FU boxes - are always installed after the glass and hot coat. So, you have no worries there. Just rout , do the standard FU install and do a gloss coat in tail area to seal.
I cringe when people say, “…just RIP OUT the old boxes…”. They should be routed out.
If in doubt, take the board to someone with the tools and jigs to do the extraction/install.