About the only way I've had success in parkerizing a part that wouldn't fit in the park tank was to mask off half of it with electrial tape and park half, neutralize with water and dry as best I could, degrease where the tape had been, mask off the parked section, then park the unparked section. You might have a slight color change where the two parkerized surfaces meet so try to have that seem where it will be hidden. I haven't had to do that since I got a nice ss tank back in the 90's though. I will still mask off parts with electrical tape though. Like the chamber end on an M1 Garand barrel which isn't supposed to be parkerized unless it's a LMR barrel or is a GI reparked gun.
There have been several guys here who have used two of the larger SS drywall mud trays from home depot. riveted together and sealed with rtv or silicone caulk. Just make sure to use steel rivets and seal them also. You can use regular steel, but you will be using some of the solution to parkerize your tank as well the first time you use it. It doesn't have to be bullet proof, it just has to hold hot water for a little while. My tank IIRC is 4"x4.5"x36" and holds 2.5 Gal of solution. The solution is good for years and used over and over if properly stored. I've found that cleaned out windshield washer fluid bottles (the ones with child proof caps) work great for storing the solution. I mark the outside of the bottles for the fluid level and make sure to fill it back up with distilled water (some use tap water) to replace the water that has boiled off. Someone here uses a specially marked plastic gas can which I think is neat also.