Mac...I like your ideas about addin' doors and built-in tackle box...I made my boxes outta treated ply and covered w/ white vinyl..If I were gonna remake w/ starboard, I'd use 3/4" sides and top...MJ's right about screwin' it all together, so ya might wanna add some inside corner-cleats to beef up the corners...screw cleats to deck, then screw box to those cleats ..West has a thru-deck butter-fly type SS fastener that I'd strongly recommend over just screwin' to the deck...thy'll be expensive comin' from West, but use 'em unless ya wanna be repairin' it later...
