I am a real advocate for keeping components cool. They last longer and are generally more stable. The heat sink provided with the BITX40 is adequate for lower power levels but bigger and better heat sinks provide for longer component lives and peace of mind. If you are going to install the transceiver in a metal box then you might consider using that enclosure as a heat sink. Whenever I have done this it has always worked very well.
In the early stages of installation I positioned the board as close to the rear of the cabinet as was convenient but it still left about 1/4” (6mm) gap between the Power Amplifier transistor (Q15) and the back panel when the original heat sink was removed. I found a small block of aluminum that fit nicely. I drilled a hole toward one end to match the hole in the tab of that IRF510.
When the mounting spacers had been attached to the BITX board (so I could tell where the transistor was located in respect to the panel) I marked and drilled the transistor's attachment hole first. I then placed the mounting bolt through the transistor, spacer block, and rear panel to temporarily hold it before marking and drilling the four holes to the attached mounting spacers of the BITX.
When mounting stuff in cabinets a bit of extra time and planning goes a long way toward preventing mistakes (and forehead slaps with cries of “OY!”). Go slow and save big washers and hole plugs.
After the board is mounted use a shouldered nylon or ceramic washer to insulate the mounting bolt from the transistor tab which is connected to the collector. Don't let it short circuit to anything. A mica or silicone pad between the spacing block and the transistor completes the insulating job while still conducting heat. If you use a mica insulator be sure to use heat sink compound on both sides of the insulator. A silicone pad does not need it.
If you cannot find a suitable aluminum block then look for any heat conductive metal. Copper, brass, even steel. I haven't tried it but I would assume that a stack of coins (with any corrosion sanded off) would work. Considering the cost of new metal stock in the hardware stores, it might be cheaper to just use money!
My BITX output transistor now runs so cool that I cannot detect any temperature increase. Just DO NOT TOUCH THE TRANSISTOR WHILE TRANSMITTING! RF burns are worse than the damage you feel from picking up the wrong end of a soldering iron. Or so I'm told.