Matt has some very good instructional videos on building his packrafts and all of the techniques will work to make a packraft. I have a few methods that I have used that may be less of a challenge.
I use a sticky stick and semi circle that works very well to hold the fabric while I weld on the belts. These are simply put, 2″ wide masking tape secured to a surface sticky side up, either flat or ached. The advantage is that you can lay your fabric on these surfaces and they will hold the fabric in place while you align the mating panel. once the panels are aligned you can lay the belt centered over the panel joint and weld about a 4-6″ section without the fear of the fabric shifting. After you weld your section you peel off the fabric and move up the panel to do the same procedure again. I find this much faster than the bowl and parchment paper.
Another I do is use a press block to press the welded joint after the weld rather than use the heat iron. I use a different type of heat system but the same idea, heat up the fabric, remove the heat and run a pressing block over the weld to set it tight. The press is nothing more than a piece of aluminum shaped like a boat with a wood block handle glued to it with JB weld. It’s only about 3″ long and 1″ wide so it’s easy to add a lot of pressure to the weld and not strain your welding tool.
Another thing I have done is to leave a window when welding up the final front and rear chamber weld. This is really simple, at a point that is over the joint junction I only weld one side of the panels for about 4″. After the “skip” on one side I continue the weld to the floor. Now I can add the sealant to the intersection at the front and rear of the raft plus some sealant at the rear tight bend on the inside. Once the sealant is dry I can go back and carefully weld the second half of the panel to the welding belt.
I also have used a small box, not much taller than a shoe box to help in the “tunnel” work. I cut a small hole in the top and taped a headlamp into it to shine down. I also cut a hole at the back (away side) to allow a flashlight to shine through.
These tricks have worked well for me but your mileage may vary. If one or more of these help I’m glad to share
Mike