Instead of putting your paper on the baseboard, or on a black card covering the baseboard, try a dark-red piece of mounting card. You can mark the required paper position in pencil (for ease of removal) at the time of framing and focussing, then easily see the marks to place your paper due to them showing up well under safelighting. No red-filter needed. Alternatively , make up a filter holder from a bit of foamboard etc. and put some rubylith in there.