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i was just gonn asuggest that boot....

Well I really wouldn't suggest the idea of the open air box (even if you do make it look like a gas tank), to easily messed with or stolen... so a cap or hard cover would be best.
Now as for the cutout itself... That is what is commonly called a blowthrough and is really quite simple for a daily driving type setup.
As you did with yours, the cab and the bed will both need to be cutout to the proper size, and you then install a cab to bed rubber boot that can be bought at most rv parts stores or online like this:

i was talking to a guy yesterday about a truck i did a year or so ago. he was asking the best way to do a cut out. and i told him i honostly couldnt tell you i can tell you how i did mine, but i can also tell you my truck didnt get driven often it was just for taking to car shows.

maybe someone who has done one can give some feed back on what i did wrong and how the best way to do it would be.

i made a box out of wood then measured it and cut out the back of the cab and the front of the bed about 1/8 inch bigger tha the box.

i welded in angle iron to reinforce the cab and bed, then slid the box into the slot.

i then took a piece of wood and cut it to the size of the back wall of the cab and glued it in place then screwed the box to it.

now in the bed of the truck i had a big wood box and i was going to put a fiberglass bed cover on.

then i got to thinking and decided it would be more fun and cheaper to use aluminum all over the box and then put a fake gas cap on the top to make it look like a fuel cell, and that would protect it from the elements.

he wants to do that to his truck, but his is a daiy driver not a show truck.

i honostly dont think my work was good enough for a daily driver, it was just an experiment on a truck i got for 200 bucks.

any feedback would be greatly apreciated...