The design and fab of the tool tray plus seat pan assembly has taken some time, but I'm almost finished. In my design the tool tray bolts down to the frame, with the seat pan then attached to the tool tray.
I made up these aluminum hooks to weld to the bottom of the seat pan. The hooks are intended to slide under the tubes I have welded into the tool tray and hold the seat pan firmly to the tool tray.
I had previously welded a solid sheet of aluminum to the underside of the seat pan to form a stable base for its mounting to the tool tray, but I decided to remove that sheet and instead use two pieces of aluminum angle to act as sub frame rails. The hooks get welded to the rails.
Here's how the rails fit onto the tool tray tubes. The sides of the tool tray fit into the groove between the rail and the hook to hold the pan from moving side to side.
I then welded the rails onto the underside of the seat pan.
To add some rigidity to the sides of the seat pan I made up end panels from flat sheet and welded them to the seat pan.
Once I got the rails and end panels welded in I did some trial fitting with the R65 and HPN tanks to find out how much of the underside hooks to trim. Here is the finished seat pan with the hooks trimmed.
Here's a back view. I just laid a string of spot welds to hold the end panels in.
I still need to arrange some kind of latch and lock mechanism. I found this nice looking draw latch
at McMaster-Carr which looks like I can mount the arm vertically on the rear end panel and the strike below on the tool tray to draw the seat pan firmly down onto to tool tray and lock it into position.
To get a feel for how much and what kinds of foam I'll need I glued on a few layers of packing foam I had on hand.
The seat pan mount is functional, but I'm not entirely satisfied with it. I don't really like the rail and hook solution I came up with. The pan is hard to put on, doesn't really fit well, and the rails aren't very strong so won't offer much theft protection. I already have a reworked design that I think will function better, but my plan is to move on to unfinished things like the seat foam and cover.