I've made quite a few custom tanks using Sullivan and similar polyethylene tanks.  It takes patience and a heavy duty soldering gun with a spade bit.  Fit parts into a simple jig.  Heat a joint until the plastic turns clear.  Then add a bit using thin strips like welding rod.

Just like learning to solder metal tanks, it does take some practice.  But the tanks are pretty cheap.  Your oval tank could probably be increased twenty percent in capacity if it was built square.  Looks like there is room to add a small wedge sump to the bottom of the tank too.

Phili,
Thanks for the information. I have seen plastic tanks welded as you describe and they did work fine, no leaks. I might try to add about 1/2" in length by cutting the back off and welding on a new one from another tank that will add the length. I will just locate the uniflow tube near the top inline with thr NVA.

Tim,
Normally the 2oz would cover the OTS pattern but for thr Drone diesel it likes fuel that is 75% ether, 25% SAE 25 mineral oil (we use mineral oil instead of castor because it calms the ether so is burns not detonate). The ether has a very wide fuel/air range similar to ethanol ( 7 - 10 to 1) so it runs a little richer ( has a very wide needle range)' Kerr based diesel is like 14 -16 to 1 (similar to gasoline, with a narrow needle range). You burn about 15 - 20% more drone fuel then kero diesel, so could use 1/2oz more fuel. The reason for the high ether is twofold, first with fixed compression it starts very easy, second it runs very cold temperature wise (the Drone has minimal cooloing fins). It still puts out tons of torque and power turning a 12-13"x8 at 6700 rpm (it turns an 11"x10 at 6700rpm also, frankly, I think it does a shaft run at 6701rpm. It is kinda the ultimate electric run.

Best,  DennisT