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i did almost the exact same thing to my sml Otter sled. was able to get away with smaller PVC since I just wanted raise the sides a little. Not to pile on top of the rails.
Great idea - may consider expanding my smaller Jet-Sled.
The "plans" I drew up are still on my work bench, but they're also pretty much hieroglyphics and utter nonsense to anyone but me. I'll share if you want to take a look. The only way to go for me was up, I thought about doing outrigger style but I was afraid it would be too tippy with the weight outside the sled. So I just went up about 8 inches to clear a bait bucket.