Xfoil is still available and there's separate discussion forums on using it. But for casual use most will agree that downloading and opening up the features of Profili2 from www.profili2.com and paying for the unlocking is money well spent for the pittance he wants. Working with the command line version of Xfoil takes a bit of a learning curve.

As for making the file up what you'll need to do is look at and replicate one of the airfoil *.dat or *.cor files using a text editor. In this case it'll be pretty darn easy since the only places that'll need anything but plus or minus .0210 (the half thickness % for your 1/8 x 3 flat plate) will be the leading and trailing edges. To make it easier for the airfoil smoothing part I'd also suggest that you don't just go directly from 0.0000 at the leading and trailing edges up to the full .0210. Instead "round" the ends a little with a step up that describes a slight radius at the corners. Something that equates to about a 1/32 radius on your 1/8 thick sheet. Otherwise the internal modelling may produce an overshoot lip at the leading and tailing edges. Use lots of coordinates at the leading and trailing edges so it confines the "curve" at these extremities and you can open up the coordinate steps in between quite a bit. Use enough to keep the surface flat and avoid the modelling algorithm from producing a wave shaped airfoil.

So yeah, there's some detail work and math or CAD time needed to produce the file but it's mostly just going to be detail work along with some cutn'paste to produce the file.

Note that you're working with percentages and not with an absolute size. You'll set the operating parameters by selecting the reynolds number. This will describe describe the speed and size in one number so that the algorithms will then "know" how the air will flow around the airfoil at any size and speed combination that matches your expected conditions for your 3 inch chord at whatever speed you choose to put into the Reynolds number equation.