Based on the textual factors, the magazine shown is very likely a reference to 月刊サンデージェネックス (Monthly Sunday Gene-X), which is also known as 月刊サンデーGX.
Let us have a closer look at the magazine in the picture:
"月刊" printed at the top, and "Monthly Yonday" printed vertically at the side of the spine of the magazine tell us that it is a monthly magazine.
The text at the bottom of the spine reads "Kougakukan", which is most likely a reference to Shougakukan (小学館), which is the publisher of the actual magazine 月刊サンデーGX.
The logo of the magazine (月刊ヨンデーFF) has close resemblance to the logo of 月刊サンデーGX. There is also the fact that there is only a difference of 1 Katakana, and how 2 English alphabet is used at the end of the name.
Logo is taken from http://sundaygx.com/
However, the logo as seen from the spine of 月刊サンデーGX magazine doesn't perfectly match up with the image from Hataraku Maou-sama. The screenshot below show how the spine of the magazine in 2006/2007 looks like (can't find a more recent image). The word 月刊 is separated from サ in the actual magazine, compared to the fictional magazine in Hataraku Maou-sama, where 月刊 is placed on top of ヨ. There might be a second reference here that I do not know of.
Image taken from http://blog-imgs-27-origin.fc2.com/n/a/0/na005/20070204-1.jpg