I do it with a Dialog Box app. In the resource editor properties for the dialog resource, set Border=None and Title Bar=False to eliminate all border elements. In OnInitDialog, use the following to resize the dialog to the entire desktop:
CRect rcDesktop;
rcDesktop.left = GetSystemMetrics(SM_XVIRTUALSCREEN);
rcDesktop.right = rcDesktop.left + GetSystemMetrics(SM_CXVIRTUALSCREEN);
rcDesktop.top = GetSystemMetrics(SM_YVIRTUALSCREEN);
rcDesktop.bottom = rcDesktop.top + GetSystemMetrics(SM_CYVIRTUALSCREEN);
MoveWindow(rcDesktop, FALSE);
This code works on multiple monitors, unlike maximizing the window.
No need to worry about making the window topmost, Windows will display it on a dedicated desktop with no other windows present.