Very sorry for the double post, that was quite weird. anyway, that little red dodge D50 was an old truck i used to have. i did the rack with angle iron on the bottom all around the front and sides. on the bottom of the rack between the bed i used a rubber weather stripping type thing which is totally flat and sticky on one side. it was about 1 1/2" if memory serves. then i put the same rubber weather stripping on the bottom of the topper. so there was actually 2 layers of rubber, and in the corners where the seams met i used a little bit of silicone, just in case. I never ever had any leaks, other than the maybe occasional drop or two at the tail gate. but that was due to the door of the topper being broken into and bent very slightly. if i had to get another one. i would lean towards fiberglass with win-doors and two separate handles on the tailgate lid that can be locked. when they broke into the old topper they just grabbed the handle and pulled bent the rods that go to the sides to "lock" it in place. i also got some of the speaker box carpet stuff and some spray glue to line the inside between the ribs. it made it sooo much better, no condensation drips on you in the middle of the night and A LOT quieter when its raining out. and im sure if you went with something even a bit more thick you would get better results. that rack i made well that you see in the pic, i would load with firewood and could walk around heck dance if i felt the need i would never worry about it. (6ft 200lbs) when i get my brand new taco and topper i will build a similar rack. but maybe out of aluminum for weight and maybe even round stock i dunno. that setup just plain worked. whatever your choice is and whatever you do, i wish you the best of luck. that was my first topper and i don't think ill ever NOT have one again. just makes it a whole lot easier when you have grocery's or whatever and its raining. good luck to you, hope you find what makes you happy without breaking the bank!
p.s. i hope my description made sense, it sure did in my head hehehe
p.s.s. you made my night, i never thought i would be able to offer advice or help? on this forum with vehicle questions. gave me a warm fuzzy. hehe