I used my old 14v Dewalt drill I purchased sometime in 2005 and one of the 18v to 20v battery adapters that Dewalt sells, I got mine with a bundle of two 2.0ah batteries and adapter for around $120

Supplies

14v Dewalt drill and adapter. I believe the screw head size is a #10 Torx and the adapter uses security ones with a pin sticking up in the center. What I do is use an automatic center punch and put the tip on the pin and push the punch until it triggers. Do that a few times and angle to smash the pin and it will either smash down far enough to use a standard Torx screwdriver or break the pin

Step 1: Going to Have to Cut the Battery Hole

The stem of the 14v battery is round and the 18v battery has the back squared off so I used a utility knife to cut mine square. I actually separated the two halves of the drill and was test fitting the adapter that way. The adapter also has protrusions on the sides that need to be cut off. Those are all to keep the 14v drill from accepting an 18v battery. For that I used a bandsaw but a hot knife or utility knife or hacksaw will work. I have a bandsaw and it's by far the fastest method so that is what I used.

Step 2: Cut the Adapter

There's six screws that hold the adapter together and two are hidden under the sticker up front. There is a little spring that's behind where the 20v battery makes it's connection so watch for that to fall out.

I held the drill halves together and traced around the top black part of the adapter and then took it to the bandsaw and trimmed inside the lines to allow for the drill edge. This took a few tries as I didn't want to cut too much off.

The orange line is where I cut the black plastic and the white is where I had to cut the yellow plastic to get it all to fit.

Step 3: A Little More Cutting on the Drill

I had to lengthen the latching notch on the drill. I used a utility knife but it would have been easier with a dremel and cut off disc. The latches on the adapter are a little further forward than the 14v drill.

Step 4: To Fit the Adapter Into the Drill

I had to loosen the two screws on the battery base of the drill and the two on the handle as well. Push the adapter in place and get it to latch and then tighten the four screws up again and the adapter will not move. If I want to use a 14v battery I'll have to loosen those screws again to remove it but I have a lot of 20v batteries and there are usually charged in some state or another whereas the 14v batteries are very old and lose charge or don't hold much charge anymore so I'll probably not use them at all.

Step 5: Done

You can see the latch is further forward and with an 18v drill there would be room to press on them and remove the adapter. The 14v drill is a touch narrower so the latches are bottomed out against the adapter housing so there is no way to press them in any further to get clearance to remove the adapter.

14v drill can now use all the 20v dewalt batteries and will spin faster and have more power than it did before. It was either this or throw the drill away as the 14v batteries I had are not holding a charge. I did rebuild one with some tenergy cells back in 2015 and that battery worked okay for a few years. This is a better solution as I don't need to carry around the old charger or batteries.