How to lengthen a boxcar filter?

Hello eveyone, I appreciated any help.

I have applied a first difference filter and a boxcar(rect filter) to my signal but it is not working. I should lengthen the spikes so as to smooth the original signal. For example, instead of (1,-1) I should apply something like (.2,.2,.2,.2,.2,-.2,-.2,-2.,-.2,-.2). Would you please direct me how should design it? Thank you in advance!

My previous code are:

% Boxcar Filter
Ts = 0.0006096; % sampling interval
T = 1; % Time 1 Sec , 
fs=1/Ts; % sampling frequency
N=4701; % 1/2 f
t=-N*Ts:Ts:(N-1)*Ts; % boxcar length
x1 = rectpuls(t, T);

and for First difference filter is:

y = filter([1,-1],1,x);  % x is my signal

[Dmitry Kaplan]

I would suggest looking here: https://www.mathworks.com/matlabcentral/fileexchange/30299-savitzky-golay-smooth-differentiation-filters-and-filter-application

[Yazan]

Why don't you just linearly interpolate, then introduces losses in the amplitude?

loss = 0.2;
y = loss * interp1([t(1), t(2)], [1 -1], t);