Nothing exotic, 5A, ~30v, low gate capacitance, low ZTC (tempco)...

Something available in the supply-chain, please.

If not, I'll go back to testing and blowing things up until I can find a "vertical mosfet" that can get the job done in a linear applicaton (load tester)

 

5A, ~30v, low gate capacitance,

Do you want a part rated at 5A or are going to put 5A through the part so you need a part rated for more?
What parts are you blowing up?
What heat sink are you using?
Blowing things up is probably a function of not using the part correctly.

 

Everything is always a "Trade-Off".

Why do You need "Low-Gate-Capacitance" for a "Load-Tester-Circuit ?

How Low is "Low" ?

What is the actual Maximum-Current that You expect to be testing with ?
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All MOSFETs will in linear mode, but no MOSFET has zero temperature coefficient. There is a single value of Vgs where the drain current does not vary with temperature, above which the drain current reduces as the temperature increases. In modern switching MOSFETs this value is above the normal operating current of the device, but for older devices it is somewhere useful.
Generally, the older the design the better it will work!
I‘d suggest something like an IRF530 or IRFP140.
If you have several in parallel, place plenty of resistance in the source to make them share current.