That said, it's pretty normal to put it under directly under the thumb, on the side you normally use for space.
I use an ErgoDash, essentially a flat version of the Dactyl Manuform, and have this layout [1]. Enter is under my right thumb, but if I hold down that key it becomes Ctrl. I could already touch-type, but it took a week or two of average use to get used to all the changes. Basic typing was quicker, but touch-typing `, [], = and the function keys took longer.
[1] http://www.keyboard-layout-editor.com/#/gists/501437fce9123e...